Existing mechanisms to mitigate packet loss due to impulse noise can be categorized as either retransmission mechanisms or forward error correction (FEC) mechanisms.
Retransmission mechanisms consist in repairing corrupted data packets by re-sending the data packets for which a retransmission request has been received. The retransmission request can be issued by the receiver that has received an irrecoverably affected data packet (such implementations are often referred to as ARQ or Automatic Repeat Request mechanisms) or alternatively can be issued by a timer waiting for the receiver to acknowledge the reception of a data packet but not receiving such acknowledgement within a predefined time from the sending of that data packet. Combinations of both, i.e. retransmission requests from the receiver for irrecoverably damaged packets and retransmission requests from a timer in or near the transmitter for unacknowledged or lost data packets, are described in literature as well.
Retransmission is typically done at the higher layers, i.e. the protocol stack layers above the physical layer such as the TCP layer or Transmission Control Protocol layer.
Retransmission has also been suggested for the physical layer, for instance in the ETSI SDSL standard contribution 054t34 from France Telecom entitled “Impulse Noise Correction in SDSL Using Retransmission Request”. In this standard contribution, it is proposed to implement at the PMD (Physical Medium Dependent) layer of an SDSL (Symmetric Digital Subscriber Line) system to record recently transmitted data segments in the SDSL transmitter. The SDSL receiver can request retransmission of a data segment that was corrupted through the occurrence of impulse noise on the copper pair by indicating the segment number of the data segment to be re-sent. The SDSL transmitter receiving a retransmission request, handles the request in priority.
Yet another implementation of retransmission is a dual layer retransmission scheme as disclosed in U.S. Pat. No. 6,931,569. Herein, the link layer of the receiver and the physical layer of the transmitter include enhancements that interact with each other for error recovery. The receiving link layer detecting lost or affected data packets sends a retransmission request towards the transmitting physical layer in order to initiate retransmission of the corrupted packet(s).
Other retransmission schemes can be found in A. S. Tanenbaum, “Computer Networks”, Fourth Ed., 2003, more particularly in section 3.3 and 3.4 thereof.
Existing retransmission mechanisms are inefficient on communication links with a high loss (due to the bandwidth expansion inherent to retransmission), a high delay (due to the latency inherent to retransmission). The known retransmission mechanisms are medium independent, meaning they are configured to operate equally on each line, irrespective of the physical conditions and/or physical configuration of the line.
Forward error correction (FEC) mechanisms are based on the calculation of a FEC code, i.e. an amount of redundant bits or bytes that are added to each data packet and can be used in the receiver's decoder to recover a limited number of transmission errors such as errors due to impulse noise. Popular FEC mechanisms are for instance Reed-Solomon encoding, Parity-Based encoding, Harris Ascent encoding, . . . . An overview of some FEC mechanisms and their efficiency to mitigate the effects of packet loss in particular for broadcast or multicast applications is given in the publication “Mitigating Packet Loss in IP Audio, Multicast Transmission” from author Jeffrey S. Pattavina. This publication can be found at the Internet via the URL:                http://www.commsdesign.com/showArticle.jhtml?articleID=18901393        
FEC mechanisms are usually implemented at the physical layer and are often combined with interleaving. By interleaving at the transmitter and de-interleaving at the receiver data bytes of consecutive FEC codewords, the effect of short duration noise or impulse noise is spread over several FEC codewords, improving the chances that the receiving decoder will be able to recover the errors induced by the impulse noise. Reed-Solomon FEC and interleaving are for instance used on ADSL (Asymmetric Digital Subscriber Line) loops. The combined configuration of the physical layer FEC encoder/decoder and interleaver/de-interleaver determines the amount of impulse noise protection or INP for a communication line. This impulse noise protection or INP can be seen as the maximum length of an impulse noise burst against which the line is protected through FEC and interleaving. Independently of the INP, also the maximum delay that can be consumed by the DSL link can be configured. Both the minimum INP and the maximum delay requirements determine the coding and interleaving parameters. An ADSL line may for instance be configured at its physical layer to have a minimum INP of 2 DMT (Discrete Multi Tone) symbols with a maximum delay of 16 milliseconds.
Retransmission techniques at higher layers may be combined with FEC techniques at the physical layer in order to mitigate the effect of impulse noise bursts that exceed the INP in length. A synergy between the two mechanisms with FEC being used to repair single packet losses and retransmission being used as an additional recovery mechanism is for instance suggested in paragraph 4.1 of RFC 2354 “Options for Repair of Streaming Media”, published at the IETF website and downloadable from the Internet via the URL:                http://www.ietf.org/rfc/rfc2354.txt?number=2354        
When an impulse noise burst exceeds the INP on a line that is protected through a FEC mechanism, a large burst of errors will occur, typically affecting several data packets. Such an error burst will introduce a correlated data packet corruption at the higher layers, i.e. multiple consecutive data packets are lost or affected at a time. As an example, an ADSL line can be considered whose physical layer interleaving function is configured to introduce an interleaving delay of 16 milliseconds. When the impulse noise protection is exceeded, an error burst of at least 16 milliseconds will occur because the interleaving function spreads consecutive corrupted bytes at least 16 milliseconds. If the ADSL line is used to convey video packets that have a length of 1500 bytes, and the downstream bit rate on the ADSL line is assumed to be 4 Mbps (megabit per second), then the number of consecutive video packets that will be corrupted is at least:ceil[16.10−3s×4.106 bits/s/(1500×8 bits/packet)]=6 packets
Thus, the configuration at the physical layer determines the burstiness of the data packet corruption. Traditional, state-of-the-art retransmission mechanisms can be combined with FEC and interleaving techniques, but they are physical media independent. In other words, such re-transmission techniques are configured equally for all lines and do not take into account differences between the lines such as the burstiness of the data corruption.
Lastly, it is acknowledged that it is known to apply FEC mechanisms at higher layers, such as e.g. the RTP (Real-Time Protocol) layer, for protection against error bursts at a lower layer. Such higher layer FEC mechanisms are also configured media independently, i.e. without taking into account physical layer differences between different lines. Such FEC mechanism can be found in IETF RFC 2733 from J. Rosenberg et al., entitled “An RTP Payload Format for Generic Forward Error Correction”. This RFC from December 1999 can be found on the Internet via URL:                http://www.ietf.org/rfc/rfc2733.txt?number=2733Another publication wherein such a FEC mechanism can be found is the Pro-MPEG Forum article “Transmission of Professional MPEG-2 Transport Streams over IP Networks”, downloadable via the following URL:        http://62.73.167.57/publications/pdf/Vid-on-IP-CoP3-r2.pdf        
Also proprietary FEC codes exist, e.g. the so-called Raptor codes which are explained at Digital Fountain's website in “DF Raptor FEC Technology” at URL:                http://www.digitalfountain.com/technology/index.cfm        
It is an object of the present patent application to disclose a device and method for mitigating effects of impulse noise on data packet transfer over a communication line, but which overcomes the above described drawbacks of existing impulse noise mitigating techniques, or combinations of such techniques.